Re-enable source maps

lib/src/codegen/js_printer.dart

Index: lib/src/codegen/js_printer.dart
diff --git a/lib/src/codegen/js_printer.dart b/lib/src/codegen/js_printer.dart
index e83548776dd33ac94b85b0dfb91f735230b2fdd1..81ea7252e3c4613097040c61755add39dbb34a07 100644
--- a/lib/src/codegen/js_printer.dart
+++ b/lib/src/codegen/js_printer.dart
@@ -72,12 +72,11 @@ class SourceMapPrintingContext extends JS.JavaScriptPrintingContext {
 
   void enterNode(JS.Node jsNode) {
     AstNode node = jsNode.sourceInformation;
-    if (node is CompilationUnit) {
-      unit = node;
+    if (node == null || node.offset == -1) return;
+    if (node.parent is CompilationUnit) {

[Jennifer Messerly, 2015/11/09 18:38:46]

We'll keep making the URI over and over, right? Since many things have the
CompilationUnit as their parent.

The idea of the old code is we'd visit the compilation unit at some point
(which, oops, should've been an invariant we asserted somewhere).

I think we lost the _visit call on the CompilationUnit. Maybe that's an
alternate way to fix this?

Alternatively, it's probably worth adding a unit != null check here, so we only
go down this path once, and then below, assert that unit must not be null. Also
exitNode needs to be changed to match the new logic here in enterNode ...

[vsm, 2015/11/09 19:15:42]

If there is a natural visit on the CompilationUnit, that seems best - but since
we're mixing parts up, I didn't think there would be.  What do you think?

I'll make the other fixes you suggested.

[vsm, 2015/11/09 19:15:42]

BTW, I tested this with Angular - seems like we have some declarations without a
unit?  The old code had a 

if (unit ==null && ... ) return;

Will dig in further.

+      unit = node.parent;
       uri = _makeRelativeUri(unit.element.source.uri);
-      return;
     }
-    if (unit == null || node == null || node.offset == -1) return;
 
     var loc = _location(node.offset);
     var name = _getIdentifier(node);