GetContentTask should catch exceptions and wrap them into AnalysisExceptions.

pkg/analyzer/test/src/task/general_test.dart

 // Copyright (c) 2015, 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 test.src.task.general_test;
 
 import 'package:analyzer/src/generated/engine.dart'
     hide AnalysisTask, GetContentTask;
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/task/general.dart';
@@ skipping 48 matching lines @@
     Source target = new TestSource();
     GetContentTask task = new GetContentTask(context, target);
     when(context.getContents(target))
         .thenReturn(new TimestampedData<String>(42, 'foo'));
     task.perform();
     expect(task.caughtException, isNull);
     expect(task.outputs, hasLength(2));
     expect(task.outputs[CONTENT], 'foo');
     expect(task.outputs[MODIFICATION_TIME], 42);
   }
+
+  void test_perform_exception() {
+    AnalysisContext context = new _MockContext();
+    Source target = new TestSource();
+    GetContentTask task = new GetContentTask(context, target);
+    when(context.getContents(target)).thenThrow('My exception!');
+    task.perform();
+    expect(task.caughtException, isNotNull);
+    expect(task.outputs, isEmpty);
+  }
 }
 
 class _MockContext extends TypedMock implements AnalysisContext {
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }