Adds a repeatable --url_mapping flag to dartanalyzer

pkg/analyzer/lib/src/generated/source.dart

Index: pkg/analyzer/lib/src/generated/source.dart
===================================================================
--- pkg/analyzer/lib/src/generated/source.dart	(revision 43787)
+++ pkg/analyzer/lib/src/generated/source.dart	(working copy)
@@ -17,7 +17,9 @@
 import 'engine.dart';
 import 'java_core.dart';
 import 'java_engine.dart';
+import 'java_io.dart' show JavaFile;
 import 'sdk.dart' show DartSdk;
+import 'source_io.dart' show FileBasedSource;
 
 /**
  * A function that is used to handle [ContentCache] entries.
@@ -165,6 +167,24 @@
   static bool isDartUri(Uri uri) => DART_SCHEME == uri.scheme;
 }
 
+class CustomUriResolver extends UriResolver {
+  final Map<String, String> _urlMappings;
+
+  CustomUriResolver(this._urlMappings);
+
+  @override
+  Source resolveAbsolute(Uri uri) {
+    String mapping = _urlMappings[uri.toString()];
+    if (mapping == null) return null;
+
+    Uri fileUri = new Uri.file(mapping);
+    if (!fileUri.isAbsolute) return null;
+
+    JavaFile javaFile = new JavaFile.fromUri(fileUri);
+    return new FileBasedSource.con1(javaFile);
+  }
+}
+
 /**
  * Instances of the class `LineInfo` encapsulate information about line and column information
  * within a source file.