First pass at asynchronous input loading in dart2js.

sdk/lib/_internal/compiler/implementation/dart2js.dart

Index: sdk/lib/_internal/compiler/implementation/dart2js.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart2js.dart b/sdk/lib/_internal/compiler/implementation/dart2js.dart
index 55812c0c6d932efd846d82de38cd91cafe254e7c..328c1d5e1af5126d9035d7edd6062aa62d817407 100644
--- a/sdk/lib/_internal/compiler/implementation/dart2js.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart2js.dart
@@ -91,7 +91,7 @@ void parseCommandLine(List<OptionHandler> handlers, List<String> argv) {
   }
 }
 
-void compile(List<String> argv) {
+Future compile(List<String> argv) {
   bool isWindows = (Platform.operatingSystem == 'windows');
   stackTraceFilePrefix = '$currentDirectory';
   Uri libraryRoot = currentDirectory;
@@ -388,10 +388,10 @@ void compile(List<String> argv) {
     return new EventSinkWrapper(writeStringSync, onDone);
   }
 
-  api.compile(uri, libraryRoot, packageRoot,
+  return api.compile(uri, libraryRoot, packageRoot,
               inputProvider.readStringFromUri, handler,
               options, outputProvider)
-      .then(compilationDone);
+            .then(compilationDone);
 }
 
 class EventSinkWrapper extends EventSink<String> {
@@ -426,11 +426,11 @@ void fail(String message) {
   exit(1);
 }
 
-void compilerMain(Options options) {
+Future compilerMain(Options options) {
   var root = uriPathToNative("/$LIBRARY_ROOT");
   List<String> argv = ['--library-root=${options.script}$root'];
   argv.addAll(options.arguments);
-  compile(argv);
+  return compile(argv);
 }
 
 void help() {
@@ -577,20 +577,22 @@ void helpAndFail(String message) {
 }
 
 void mainWithErrorHandler(Options options) {
-  try {
-    compilerMain(options);
-  } catch (exception, trace) {
+  compilerMain(options).catchError((exception) {
     try {
       print('Internal error: $exception');
     } catch (ignored) {
       print('Internal error: error while printing exception');
     }
+    
     try {
-      print(trace);
+      var trace = getAttachedStackTrace(exception);
+      if (trace != null) {
+        print(trace);
+      }
     } finally {
       exit(253); // 253 is recognized as a crash by our test scripts.
     }
-  }
+  });
 }
 
 void main() {