Handle 404 fetch better.

runtime.ts

@@ -146,10 +146,14 @@ export function resolveModule(
  util.assert(moduleSpecifier != null && moduleSpecifier.length > 0);
  // We ask golang to sourceCodeFetch. It will load the sourceCode and if
  // there is any outputCode cached, it will return that as well.
- const { filename, sourceCode, outputCode } = os.codeFetch(
- moduleSpecifier,
- containingFile
- );
+ let fetchResponse;
+ try {
+ fetchResponse = os.codeFetch(moduleSpecifier, containingFile);
+ } catch(e) {
+ // TODO Only catch "no such file or directory" errors. Need error codes.
+ return null;
+ }
+ const { filename, sourceCode, outputCode } = fetchResponse;
  if (sourceCode.length === 0) {
  return null;
  }
@@ -165,9 +169,13 @@ export function resolveModule(
 function resolveModuleName(
  moduleSpecifier: string,
  containingFile: string
-): string {
+): string | undefined {
  const mod = resolveModule(moduleSpecifier, containingFile);
- return mod.fileName;
+ if (mod) {
+ return mod.fileName;
+ } else {
+ return undefined;
+ }
 }
 
 function execute(fileName: string, outputCode: string): void {
@@ -308,6 +316,9 @@ class TypeScriptHost implements ts.LanguageServiceHost {
  resolvedFileName = resolveModuleName("typescript.d.ts", "/$asset$/");
  } else {
  resolvedFileName = resolveModuleName(name, containingFile);
+ if (resolvedFileName == null) {
+ return undefined;
+ }
  }
  const isExternalLibraryImport = false;
  return { resolvedFileName, isExternalLibraryImport };