Re-triage mirror test failures on the analyzer.

tests/lib/mirrors/method_mirror_source_test.dart

 // Copyright (c) 2013, 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.
 
 import "dart:mirrors";
 
 import "package:expect/expect.dart";
 
 expectSource(Mirror mirror, String source) {
+  MethodMirror methodMirror;
   if (mirror is ClosureMirror) {
-    mirror = mirror.function;
+    methodMirror = mirror.function;
+  } else {
+    methodMirror = mirror as MethodMirror;
   }
-  Expect.isTrue(mirror is MethodMirror);
-  Expect.equals(mirror.source, source);
+  Expect.isTrue(methodMirror is MethodMirror);
+  Expect.equals(methodMirror.source, source);
 }
 
 foo1() {}
 doSomething(e) => e;
 
 int get x => 42;
 set x(value) { }
 
 class S {}
 
@@ skipping 64 matching lines @@
   // Closures
   expectSource(reflect((){}), "(){}");
   expectSource(reflect((x,y,z) { return x*y*z; }), "(x,y,z) { return x*y*z; }");
   expectSource(reflect((e) => doSomething(e)), "(e) => doSomething(e)");
 
   namedClosure(x,y,z) => 1;
   var a = () {};
   expectSource(reflect(namedClosure), "namedClosure(x,y,z) => 1;");
   expectSource(reflect(a), "() {}");
 }