fixes #201, add support for custom URL mapping

lib/src/analysis_context.dart

Index: lib/src/analysis_context.dart
diff --git a/lib/src/analysis_context.dart b/lib/src/analysis_context.dart
index 26677b19fd293702adafa9e8c641091f3c71caaa..4b8d3106eadf347c35184985d7e65c7b5792eb23 100644
--- a/lib/src/analysis_context.dart
+++ b/lib/src/analysis_context.dart
@@ -34,7 +34,11 @@ AnalysisContext createAnalysisContext(CompilerOptions options,
       ? createMockSdkResolver(mockSdkSources)
       : createSdkPathResolver(options.dartSdkPath);
 
-  var resolvers = [sdkResolver];
+  var resolvers = [];
+  if (options.customUrlMappings.isNotEmpty) {
+    resolvers.add(new CustomUriResolver(options.customUrlMappings));
+  }
+  resolvers.add(sdkResolver);
   if (options.useImplicitHtml) {
     resolvers.add(_createImplicitEntryResolver(options));
   }