Port two completion contributors for use by plugins

pkg/analyzer_plugin/test/support/abstract_context.dart

Index: pkg/analyzer_plugin/test/support/abstract_context.dart
diff --git a/pkg/analyzer_plugin/test/support/abstract_context.dart b/pkg/analyzer_plugin/test/support/abstract_context.dart
index 8d6329ba04d6c8b2f21f177a941138ceff22b9b6..9ec849b040287c1955ad26fe10346b09a494ad18 100644
--- a/pkg/analyzer_plugin/test/support/abstract_context.dart
+++ b/pkg/analyzer_plugin/test/support/abstract_context.dart
@@ -56,6 +56,11 @@ class AbstractContextTest {
 
   AnalysisDriver get driver => _driver;
 
+  /**
+   * Return `true` if strong mode should be enabled for this test.
+   */
+  bool get enableStrongMode => false;
+
   Source addMetaPackageSource() => addPackageSource(
       'meta',
       'meta.dart',
@@ -114,6 +119,8 @@ class Required {
         [new DartUriResolver(sdk), packageResolver, resourceResolver]);
     PerformanceLog log = new PerformanceLog(_logBuffer);
     AnalysisDriverScheduler scheduler = new AnalysisDriverScheduler(log);
+    AnalysisOptionsImpl options = new AnalysisOptionsImpl()
+      ..strongMode = enableStrongMode;
     _driver = new AnalysisDriver(
         scheduler,
         log,
@@ -122,7 +129,7 @@ class Required {
         _fileContentOverlay,
         null,
         sourceFactory,
-        new AnalysisOptionsImpl());
+        options);
     scheduler.start();
     AnalysisEngine.instance.logger = PrintLogger.instance;
   }