Dev compiler debugger related tweaks.

pkg/dev_compiler/lib/src/compiler/compiler.dart

Index: pkg/dev_compiler/lib/src/compiler/compiler.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/compiler.dart b/pkg/dev_compiler/lib/src/compiler/compiler.dart
index e90d37259c18e5796344975881da371aebaac522..b34ce40b496332fc444f9a9172f107bfdaa1e6e6 100644
--- a/pkg/dev_compiler/lib/src/compiler/compiler.dart
+++ b/pkg/dev_compiler/lib/src/compiler/compiler.dart
@@ -602,13 +602,13 @@ Map placeSourceMap(
 
 /// Cleanup the dart_sdk source map.
 ///
-/// Strip out files that should not be included in the sdk sourcemap as they
-/// are implementation details that would just confuse users.
 /// Normalize sdk urls to use "dart:" for more understandable stack traces.
 Map cleanupSdkSourcemap(Map sourceMap) {
   var map = new Map.from(sourceMap);
-  map['sources'] = map['sources']
-      .map((url) => url.contains('/_internal/') ? null : url)
-      .toList();
+  map['sources'] = map['sources'].map((url) {
+    var urlPrefix = '../../../gen/patched_sdk/lib/';
+    if (!url.startsWith(urlPrefix)) throw new Error('Unexpected sdk url: $url');
+    return 'dart:${url.substring(urlPrefix.length)}';
+  }).toList();
   return map;
 }