Reinsert (and test) handling of exceptions from user code.

pkg/compiler/lib/src/apiimpl.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library leg_apiimpl;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:package_config/packages.dart';
@@ skipping 260 matching lines @@
       callUserHandler(
           message, span.uri, span.begin, span.end, '$message', kind);
     }
   }
 
   bool get isMockCompilation =>
       mockableLibraryUsed && options.allowMockCompilation;
 
   void callUserHandler(Message message, Uri uri, int begin, int end,
       String text, api.Diagnostic kind) {
-    userHandlerTask.measure(() {
-      handler.report(message, uri, begin, end, text, kind);
-    });
+    try {
+      userHandlerTask.measure(() {
+        handler.report(message, uri, begin, end, text, kind);
+      });
+    } catch (ex, s) {
+      reportCrashInUserCode('Uncaught exception in diagnostic handler', ex, s);
+      rethrow;
+    }
   }
 
   Future callUserProvider(Uri uri) {
-    return userProviderTask.measure(() => provider.readFromUri(uri));
+    try {
+      return userProviderTask.measure(() => provider.readFromUri(uri));
+    } catch (ex, s) {
+      reportCrashInUserCode('Uncaught exception in input provider', ex, s);
+      rethrow;
+    }
   }
 
   Future<Packages> callUserPackagesDiscovery(Uri uri) {
-    return userPackagesDiscoveryTask
-        .measure(() => options.packagesDiscoveryProvider(uri));
+    try {
+      return userPackagesDiscoveryTask
+          .measure(() => options.packagesDiscoveryProvider(uri));
+    } catch (ex, s) {
+      reportCrashInUserCode('Uncaught exception in package discovery', ex, s);
+      rethrow;
+    }
   }
 
   Uri resolvePatchUri(String libraryName) {
     return backend.resolvePatchUri(libraryName, options.platformConfigUri);
   }
 }
 
 class _Environment implements Environment {
   final Map<String, String> definitions;
 
@@ skipping 31 matching lines @@
   }
 }
 
 /// For every 'dart:' library, a corresponding environment variable is set
 /// to "true". The environment variable's name is the concatenation of
 /// this prefix and the name (without the 'dart:'.
 ///
 /// For example 'dart:html' has the environment variable 'dart.library.html' set
 /// to "true".
 const String _dartLibraryEnvironmentPrefix = 'dart.library.';