Make some VM libraries patch cleanly using the analyzer.

sdk/lib/mirrors/mirrors.dart

Index: sdk/lib/mirrors/mirrors.dart
diff --git a/sdk/lib/mirrors/mirrors.dart b/sdk/lib/mirrors/mirrors.dart
index 206410a7ec042cea7d8c89665c90085f8c600c44..3aa88f4c24bcc0295518ac724b0471ea03ed58b5 100644
--- a/sdk/lib/mirrors/mirrors.dart
+++ b/sdk/lib/mirrors/mirrors.dart
@@ -87,10 +87,7 @@ abstract class MirrorSystem {
    *
    * If no unique library exists, an error is thrown.
    */
-  LibraryMirror findLibrary(Symbol libraryName) {

[Siggi Cherem (dart-lang), 2016/12/15 00:31:46]

I was thinking whether there will be much of an impact if we take the vm
implementation here.

I like that the vm patch has better error messages. The risk is that those error
messages call `getName` which can cause more bloat (although if you are already
using `findLibrary` we are likely already in a place with a lot of bloat
regardless).

My current inclination is to keep what you did here, but I just wanted to bring
it up a possible alternative.

-    return libraries.values.singleWhere(
-        (library) => library.simpleName == libraryName);
-  }
+  external LibraryMirror findLibrary(Symbol libraryName);
 
   /**
    * A mirror on the isolate associated with this [MirrorSystem].