Add empty mapping entries for the ranges of generated JavaScript that don't map to any Dart source …

lib/compiler/implementation/code_buffer.dart

Index: lib/compiler/implementation/code_buffer.dart
diff --git a/lib/compiler/implementation/code_buffer.dart b/lib/compiler/implementation/code_buffer.dart
index 3c3053e429942e4cfa4f7018efbf1998d6576e7b..02c04d9607c3e94c1a7ef93ff00fa8873bc1a186 100644
--- a/lib/compiler/implementation/code_buffer.dart
+++ b/lib/compiler/implementation/code_buffer.dart
@@ -6,6 +6,7 @@ class CodeBuffer implements StringBuffer {
   StringBuffer buffer;
   List<SourceLocation> sourceLocations;
   int lastBufferOffset = 0;
+  int mappedRangeCounter = 0;
 
   CodeBuffer()
       : buffer = new StringBuffer(),
@@ -25,6 +26,7 @@ class CodeBuffer implements StringBuffer {
     if (object is CodeBuffer) {
       return addBuffer(object);
     }
+    if (mappedRangeCounter == 0) setSourceLocation(null, null);
     buffer.add(object.toString());
     return this;
   }
@@ -67,6 +69,15 @@ class CodeBuffer implements StringBuffer {
     return buffer.toString();
   }
 
+  void beginMappedRange() {
+    ++mappedRangeCounter;
+  }
+
+  void endMappedRange() {
+    assert(mappedRangeCounter > 0);
+    --mappedRangeCounter;
+  }
+
   void setSourceLocation(Element element, Token token) {
     int offsetDelta = buffer.length - lastBufferOffset;
     sourceLocations.add(new SourceLocation(element, token, offsetDelta));