Don't emit repeated null SourceLocations

sdk/lib/_internal/compiler/implementation/code_buffer.dart

Index: sdk/lib/_internal/compiler/implementation/code_buffer.dart
diff --git a/sdk/lib/_internal/compiler/implementation/code_buffer.dart b/sdk/lib/_internal/compiler/implementation/code_buffer.dart
index 4662eb71e0e54c1bc1a9a0ee233f484cbdaf0f7b..fcde4affa9c353931c7c632ea04ca17144c69a00 100644
--- a/sdk/lib/_internal/compiler/implementation/code_buffer.dart
+++ b/sdk/lib/_internal/compiler/implementation/code_buffer.dart
@@ -104,6 +104,9 @@ class CodeBuffer implements StringBuffer {
   }
 
   void setSourceLocation(var sourcePosition) {
+    if (sourcePosition == null) {
+      if (markers.length > 0 && markers.last.sourcePosition == null) return;
+    }
     int offsetDelta = buffer.length - lastBufferOffset;
     markers.add(new CodeBufferMarker(offsetDelta, sourcePosition));
     lastBufferOffset = buffer.length;