Improve handling of locations not from the uris the source map is for.

lib/parser.dart

Index: lib/parser.dart
diff --git a/lib/parser.dart b/lib/parser.dart
index c651f89ff11748fbe954655495274cc0885c767e..3ae25967d462fffa1943bd8a97a3d64684d3a7ab 100644
--- a/lib/parser.dart
+++ b/lib/parser.dart
@@ -213,7 +213,14 @@ class MappingBundle extends Mapping {
     if (_mappings.containsKey(name)) {
       return _mappings[name].spanFor(line, column, files: files, uri: name);
     }
-    return null;
+    // The uri doesn't have a source map so create a SourceMapSpan matching
+    // the input location as well as possible. Fake the file offset because

[Siggi Cherem (dart-lang), 2016/12/09 16:38:38]

nits:
- input and output is a bit confusing here. it's the input to this function, but
normally we refer to is as the output file.
- consider splitting the discussion of fake offsets to a separate paragraph.

One possible suggestion:

// Note: when there is no source map for an uri, this behaves like an
// identity function, returning the requested location as the result.

// Create a mock offset for the output location. We compute it in terms
// of the input line and column to minimize the chances that two different
// line and column locations are mapped to the same offset.

[Jacob, 2016/12/09 17:18:03]

Done.

+    // we don't really know it. This fake offset avoids locations with the
+    // same offset but different lines and columns.
+    var offset = line * 1000000000 + column;

[Siggi Cherem (dart-lang), 2016/12/09 16:38:38]

wow, 1 billion! that's a very long line :)

Can we make it smaller? I think the chances of this being an issue are pretty
low if you use something like 1000. It would be nice to use a number that is
likely going to fit in 32 bit.

[Jacob, 2016/12/09 17:18:03]

Switched to a million. Note that SourceMapSpan does identity comparisons based
on just the offset so it is a bit scary that there could be collisions.

+    var location = new SourceLocation(offset,
+        line: line, column: column, sourceUrl: Uri.parse(uri));
+    return new SourceMapSpan(location, location, "");
   }
 }