Better DDC sourcemap generation for lambdas

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

Index: pkg/dev_compiler/lib/src/compiler/source_map_printer.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/source_map_printer.dart b/pkg/dev_compiler/lib/src/compiler/source_map_printer.dart
index 3320e8d2af92ee2b899b0a2e1711a3a6074d69b3..ac70232f1198f0ae65c201fa10c6270af549f185 100644
--- a/pkg/dev_compiler/lib/src/compiler/source_map_printer.dart
+++ b/pkg/dev_compiler/lib/src/compiler/source_map_printer.dart
@@ -76,6 +76,10 @@ class SourceMapPrintingContext extends JS.SimpleJavaScriptPrintingContext {
 
   void _mark(int offset, [String identifier]) {
     var loc = unit.lineInfo.getLocation(offset);
+    var next = unit.lineInfo.getLocation(offset + 1);

[vsm, 2017/04/11 18:39:51]

Can you add a comment here to explain?  This looks a bit magical.  :-)

[Alan Knight, 2017/04/11 20:32:06]

Done.

+    if (next.lineNumber == loc.lineNumber + 1) {
+      loc = next;
+    }
     sourceMap.addLocation(
         new SourceLocation(offset,
             sourceUrl: sourcePath,