Fix issue with runtime lib generation.

tools/ts_library_builder/ast_util.ts

@@ -58,6 +58,14 @@ export function addVariableDeclaration(
   });
 }
 
+/** Copy one source file to the end of another source file. */
+export function appendSourceFile(
+  sourceFile: SourceFile,
+  targetSourceFile: SourceFile
+): void {
+  targetSourceFile.addStatements(`\n${sourceFile.print()}`);
+}
+
 /** Check diagnostics, and if any exist, exit the process */
 export function checkDiagnostics(project: Project, onlyFor?: string[]) {
   const program = project.getProgram();

tools/ts_library_builder/build_library.ts

@@ -13,6 +13,7 @@ import {
   addInterfaceProperty,
   addSourceComment,
   addVariableDeclaration,
+  appendSourceFile,
   checkDiagnostics,
   flattenNamespace,
   getSourceComment,
@@ -421,6 +422,16 @@ export function main({
     console.log(`Merged "globals" into global scope.`);
   }
 
+  // Since we flatten the namespaces, we don't attempt to import `text-encoding`
+  // so we then need to concatenate that onto the `libDts` so it can stand on
+  // its own.
+  const textEncodingSourceFile = outputProject.getSourceFileOrThrow(
+    textEncodingFilePath
+  );
+  appendSourceFile(textEncodingSourceFile, libDTs);
+  // Removing it from the project so we know the libDTs can stand on its own.
+  outputProject.removeSourceFile(textEncodingSourceFile);
+
   // Add the preamble
   libDTs.insertStatements(0, libPreamble);