Hide ClosureMirror.findInContext behind a flag for 1.0.

runtime/lib/mirrors_impl.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.
 
 // VM-specific implementation of the dart:mirrors library.
 
 import "dart:collection";
 
 final emptyList = new UnmodifiableListView([]);
 final emptyMap = new _UnmodifiableMapView({});
@@ skipping 451 matching lines @@
                         _unwrapAsyncNamed(namedArguments));
     });
   }
 
   InstanceMirror findInContext(Symbol name, {ifAbsent: null}) {
     List<String> parts = _n(name).split(".").toList(growable: false);
     if (parts.length > 3) {
       throw new ArgumentError("Invalid symbol: ${name}");
     }
     List tuple = _computeFindInContext(_reflectee, parts);
+    if (tuple.length == 0) {
+      throw new UnsupportedError(
+          "ClosureMirror.findInContext not yet supported");
+    }
     if (tuple[0]) {
       return reflect(tuple[1]);
     }
     return ifAbsent == null ? null : ifAbsent();
   }
 
   String toString() => "ClosureMirror on '${Error.safeToString(_reflectee)}'";
 
   static _apply(arguments, argumentNames)
       native 'ClosureMirror_apply';
@@ skipping 1093 matching lines @@
     if (typeMirror == null) {
       typeMirror = makeLocalTypeMirror(key);
       _instanitationCache[key] = typeMirror;
       if (typeMirror is ClassMirror && !typeMirror._isGeneric) {
         _declarationCache[key] = typeMirror;
       }
     }
     return typeMirror;
   }
 }