Fix recently introduced strong mode errors

pkg/analyzer/lib/src/context/context.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 analyzer.src.context.context;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 1566 matching lines @@
   }
 
   /**
    * Return a [CompilationUnit] for the given library and unit sources, which
    * can be incrementally resolved.
    */
   CompilationUnit _getIncrementallyResolvableUnit(
       Source librarySource, Source unitSource) {
     LibrarySpecificUnit target =
         new LibrarySpecificUnit(librarySource, unitSource);
-    for (ResultDescriptor result in [
+    for (ResultDescriptor<CompilationUnit> result in [
       RESOLVED_UNIT,
       RESOLVED_UNIT12,
       RESOLVED_UNIT11,
       RESOLVED_UNIT10,
       RESOLVED_UNIT9,
       RESOLVED_UNIT8
     ]) {
       CompilationUnit unit = getResult(target, result);
       if (unit != null) {
         return unit;
@@ skipping 714 matching lines @@
     }
     DartSdk sdk = factory.dartSdk;
     if (sdk == null) {
       throw new IllegalArgumentException(
           "The source factory for an SDK analysis context must have a DartUriResolver");
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }