Fix issue with caching source files

Author: kitsonk

js/compiler.ts

@@ -264,8 +264,9 @@ class Host implements ts.CompilerHost {
     const sourceFile = fetchSourceFile(specifier, referrer);
     assert(sourceFile.moduleName != null);
     const { moduleName } = sourceFile;
-    assert(!(moduleName! in this._sourceFileCache));
-    this._sourceFileCache[moduleName!] = sourceFile;
+    if (!(moduleName! in this._sourceFileCache)) {
+      this._sourceFileCache[moduleName!] = sourceFile;
+    }
     return sourceFile;
   }
 

js/type_directives.ts

@@ -49,6 +49,7 @@ export function parseTypeDirectives(
     return;
   }
 
+  // collect all the directives in the file and their start and end positions
   const directives: DirectiveInfo[] = [];
   let maybeMatch: RegExpExecArray | null = null;
   while ((maybeMatch = typeDirectiveRegEx.exec(sourceCode))) {
@@ -64,6 +65,8 @@ export function parseTypeDirectives(
     return;
   }
 
+  // work from the last directive backwards for any `import`/`export` statements
+  // adding their module specifiers to
   directives.reverse();
   const directiveRecords: Record<string, string> = {};
   for (const { path, start, end } of directives) {