Better DDC sourcemap generation for lambdas

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
 import 'package:source_maps/source_maps.dart' hide Printer;
 import 'package:source_span/source_span.dart' show SourceLocation;
 
 import '../js_ast/js_ast.dart' as JS;
@@ skipping 24 matching lines @@
         _column = 0;
       } else {
         _column++;
       }
     }
     super.emit(code);
   }
 
   void enterNode(JS.Node jsNode) {
     AstNode node = jsNode.sourceInformation;
-    if (node == null || node.offset == -1) return;
+    if (node == null || node.offset == -1 || node.isSynthetic) return;
     if (unit == null) {
       // This is a top-level declaration.  Note: consecutive top-level
       // declarations may come from different compilation units due to
       // parts.
       _currentTopLevelDeclaration = node;
       unit = node.getAncestor((n) => n is CompilationUnit);
       sourcePath =
           resolutionMap.elementDeclaredByCompilationUnit(unit).source.fullName;
     }
-
+    if (node is MethodDeclaration) return;
     _mark(node.offset, _getIdentifier(node));
   }
 
   void exitNode(JS.Node jsNode) {
     AstNode node = jsNode.sourceInformation;
     if (unit == null || node == null || node.offset == -1) return;
 
     // TODO(jmesserly): in many cases marking the end will be unnecessary.
-    _mark(node.end);
+    if (node is! MethodDeclaration) {

[Jennifer Messerly, 2017/04/12 16:11:23]

Add a comment on why we're skipping MethodDeclaration in particular?

+      _mark(node.end);
+    }
 
     if (identical(node, _currentTopLevelDeclaration)) {
       unit = null;
       sourcePath = null;
       _currentTopLevelDeclaration == null;
     }
   }
 
   // TODO(jmesserly): prefix identifiers too, if they map to a named element.
   String _getIdentifier(AstNode node) =>
       node is SimpleIdentifier ? node.name : null;
 
   void _mark(int offset, [String identifier]) {
     var loc = unit.lineInfo.getLocation(offset);
+    // Chrome Devtools wants a mapping for the beginning of
+    // a line, so bump locations at the end of a line to the beginning of
+    // the next line.
+    var next = unit.lineInfo.getLocation(offset + 1);
+    if (next.lineNumber == loc.lineNumber + 1) {
+      loc = next;
+    }
     sourceMap.addLocation(
         new SourceLocation(offset,
             sourceUrl: sourcePath,
             line: loc.lineNumber - 1,
             column: loc.columnNumber - 1),
         new SourceLocation(buffer.length, line: _line, column: _column),
         identifier);
   }
 }
 
 const int _LF = 10;
 const int _CR = 13;