diff --git a/Makefile b/Makefile
index 25416012b2..93791200ea 100644
--- a/Makefile
+++ b/Makefile
@@ -13,7 +13,7 @@ msg.pb.js: msg.proto node_modules
 msg.pb.d.ts: msg.pb.js node_modules
 	./node_modules/.bin/pbts -o msg.pb.d.ts msg.pb.js
 
-dist/main.js: main.ts compiler.ts fs.ts util.ts msg.pb.js msg.pb.d.ts node_modules
+dist/main.js: main.ts compiler.ts os.ts util.ts msg.pb.js msg.pb.d.ts node_modules
 	./node_modules/.bin/tsc --noEmit # Only for type checking.
 	./node_modules/.bin/parcel build --out-dir=dist/ --no-minify main.ts
 
diff --git a/compiler.ts b/compiler.ts
index fdd9dc997a..7f889b30c7 100644
--- a/compiler.ts
+++ b/compiler.ts
@@ -1,23 +1,32 @@
 import * as ts from "typescript";
 import { assert, globalEval } from "./util";
-import { readFileSync } from "./fs";
+import { exit, readFileSync } from "./os";
+import * as path from "path";
 
 export function compile(cwd: string, inputFn: string): void {
   const options: ts.CompilerOptions = {
-    "allowJs": true,
-    "outFile": "out.js",
+    allowJs: true,
+    outFile: "out.js"
   };
   const host = new CompilerHost(cwd);
 
-  let program = ts.createProgram([inputFn], options, host);
+  const inputExt = path.extname(inputFn);
+  if (!EXTENSIONS.includes(inputExt)) {
+    console.error(`Bad file name extension for input "${inputFn}"`);
+    exit(1);
+  }
+
+  const program = ts.createProgram([inputFn], options, host);
   //let sourceFiles = program.getSourceFiles();
   //console.log("rootFileNames", program.getRootFileNames());
 
-  let emitResult = program.emit();
+  const emitResult = program.emit();
   assert(!emitResult.emitSkipped);
   //console.log("emitResult", emitResult);
 }
 
+const EXTENSIONS = [".ts", ".js"];
+
 export class CompilerHost {
   constructor(public cwd: string) {}
 
@@ -101,9 +110,24 @@ export class CompilerHost {
     moduleNames: string[],
     containingFile: string,
     reusedNames?: string[]
-  ): (ts.ResolvedModule | undefined)[] {
-    console.log("resolveModuleNames", moduleNames);
-    return [];
+  ): Array<ts.ResolvedModule | undefined> {
+    console.log("resolveModuleNames", { moduleNames, reusedNames });
+    return moduleNames.map((name: string) => {
+      if (
+        name.startsWith("/") ||
+        name.startsWith("http://") ||
+        name.startsWith("https://")
+      ) {
+        throw Error("Non-relative imports not yet supported.");
+      } else {
+        // Relative import.
+        console.log("relative import", { containingFile, name });
+        const containingDir = path.dirname(containingFile);
+        const resolvedFileName = path.join(containingDir, name);
+        const isExternalLibraryImport = false;
+        return { resolvedFileName, isExternalLibraryImport };
+      }
+    });
   }
 
   fileExists(fileName: string): boolean {
diff --git a/fs.ts b/fs.ts
deleted file mode 100644
index a3d4c95195..0000000000
--- a/fs.ts
+++ /dev/null
@@ -1,18 +0,0 @@
-import { main as pb } from "./msg.pb";
-import { TextDecoder } from "text-encoding";
-
-export function readFileSync(filename: string): string {
-  const msg = pb.Msg.fromObject({
-    kind: pb.Msg.MsgKind.READ_FILE_SYNC,
-    path: filename
-  });
-  const ui8 = pb.Msg.encode(msg).finish();
-  const ab = ui8.buffer.slice(ui8.byteOffset, ui8.byteOffset + ui8.byteLength);
-  const resBuf = V8Worker2.send(ab as ArrayBuffer);
-  const res = pb.Msg.decode(new Uint8Array(resBuf));
-  if (res.error != null && res.error.length > 0) {
-    throw Error(res.error);
-  }
-  const decoder = new TextDecoder("utf8");
-  return decoder.decode(res.data);
-}
diff --git a/main.go b/main.go
index e634a6abe2..6e38d5f95b 100644
--- a/main.go
+++ b/main.go
@@ -31,9 +31,13 @@ func recv(buf []byte) []byte {
 	switch msg.Kind {
 	case Msg_READ_FILE_SYNC:
 		return ReadFileSync(msg.Path)
+	case Msg_EXIT:
+		os.Exit(int(msg.Code))
 	default:
 		panic("Unexpected message")
 	}
+
+	return nil
 }
 
 func loadAsset(w *v8worker2.Worker, path string) {
diff --git a/main.ts b/main.ts
index bd1b13ed5e..6e01557595 100644
--- a/main.ts
+++ b/main.ts
@@ -2,7 +2,6 @@ import { main as pb } from "./msg.pb";
 import "./util";
 import { compile } from "./compiler";
 
-
 function start(cwd: string, argv: string[]): void {
   // TODO parse arguments.
   const inputFn = argv[1];
diff --git a/msg.proto b/msg.proto
index df0b7c102e..fcbd9bcbce 100644
--- a/msg.proto
+++ b/msg.proto
@@ -7,6 +7,7 @@ message Msg {
     START = 0;
     READ_FILE_SYNC = 1;
     DATA_RESPONSE = 2;
+    EXIT = 3;
   }
   MsgKind kind = 10;
 
@@ -20,4 +21,7 @@ message Msg {
   // DATA_RESPONSE
 	bytes data = 30;
   string error = 31;
+
+  // EXIT
+  int32 code = 40;
 }
diff --git a/os.ts b/os.ts
new file mode 100644
index 0000000000..ca97e98e94
--- /dev/null
+++ b/os.ts
@@ -0,0 +1,41 @@
+import { main as pb } from "./msg.pb";
+import { TextDecoder } from "text-encoding";
+
+// TODO move this to types.ts
+type TypedArray = Uint8Array | Float32Array | Int32Array;
+
+export function exit(code = 0): void {
+  sendMsgFromObject({
+    kind: pb.Msg.MsgKind.EXIT,
+    code
+  });
+}
+
+export function readFileSync(filename: string): string {
+  const res = sendMsgFromObject({
+    kind: pb.Msg.MsgKind.READ_FILE_SYNC,
+    path: filename
+  });
+  if (res.error != null && res.error.length > 0) {
+    throw Error(res.error);
+  }
+  const decoder = new TextDecoder("utf8");
+  return decoder.decode(res.data);
+}
+
+function typedArrayToArrayBuffer(ta: TypedArray): ArrayBuffer {
+  const ab = ta.buffer.slice(ta.byteOffset, ta.byteOffset + ta.byteLength);
+  return ab as ArrayBuffer;
+}
+
+function sendMsgFromObject(obj: pb.IMsg): null | pb.Msg {
+  const msg = pb.Msg.fromObject(obj);
+  const ui8 = pb.Msg.encode(msg).finish();
+  const ab = typedArrayToArrayBuffer(ui8);
+  const resBuf = V8Worker2.send(ab);
+  if (resBuf != null && resBuf.byteLength > 0) {
+    return pb.Msg.decode(new Uint8Array(resBuf));
+  } else {
+    return null;
+  }
+}
diff --git a/testdata/hello_world.js b/testdata/001_hello.js
similarity index 100%
rename from testdata/hello_world.js
rename to testdata/001_hello.js
diff --git a/testdata/hello_world.js.out b/testdata/001_hello.js.out
similarity index 100%
rename from testdata/hello_world.js.out
rename to testdata/001_hello.js.out
diff --git a/testdata/hello_world.ts b/testdata/002_hello.ts
similarity index 100%
rename from testdata/hello_world.ts
rename to testdata/002_hello.ts
diff --git a/testdata/hello_world.ts.out b/testdata/002_hello.ts.out
similarity index 100%
rename from testdata/hello_world.ts.out
rename to testdata/002_hello.ts.out
diff --git a/testdata/003_relative_import.ts b/testdata/003_relative_import.ts
new file mode 100644
index 0000000000..01d5d7faac
--- /dev/null
+++ b/testdata/003_relative_import.ts
@@ -0,0 +1,3 @@
+import { printHello } from "./subdir/print_hello.ts";
+
+printHello();
diff --git a/testdata/003_relative_import.ts.out b/testdata/003_relative_import.ts.out
new file mode 100644
index 0000000000..e965047ad7
--- /dev/null
+++ b/testdata/003_relative_import.ts.out
@@ -0,0 +1 @@
+Hello
diff --git a/testdata/subdir/print_hello.ts b/testdata/subdir/print_hello.ts
new file mode 100644
index 0000000000..7ecce50405
--- /dev/null
+++ b/testdata/subdir/print_hello.ts
@@ -0,0 +1,3 @@
+export function printHello(): void {
+  console.log("Hello");
+}
diff --git a/util.ts b/util.ts
index f5e5e847e6..7a66669426 100644
--- a/util.ts
+++ b/util.ts
@@ -13,18 +13,28 @@ const print = V8Worker2.print;
 _global["console"] = {
   // tslint:disable-next-line:no-any
   log(...args: any[]): void {
-    const out: string[] = [];
-    for (const a of args) {
-      if (typeof a === "string") {
-        out.push(a);
-      } else {
-        out.push(JSON.stringify(a));
-      }
-    }
-    print(out.join(" "));
+    print(stringifyArgs(args));
+  },
+
+  // tslint:disable-next-line:no-any
+  error(...args: any[]): void {
+    print("ERROR: " + stringifyArgs(args));
   }
 };
 
+// tslint:disable-next-line:no-any
+function stringifyArgs(args: any[]): string {
+  const out: string[] = [];
+  for (const a of args) {
+    if (typeof a === "string") {
+      out.push(a);
+    } else {
+      out.push(JSON.stringify(a));
+    }
+  }
+  return out.join(" ");
+}
+
 export function assert(cond: boolean, msg = "") {
   if (!cond) {
     throw Error("Assertion failed. " + msg);