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

test/parser_test.dart

Index: test/parser_test.dart
diff --git a/test/parser_test.dart b/test/parser_test.dart
index a317f7e6370382578ba75a645a78f34b972c83df..4b2d947134a7d6c82553e1700332f82f44dd3388 100644
--- a/test/parser_test.dart
+++ b/test/parser_test.dart
@@ -250,7 +250,7 @@ main() {
       expect(mapping.toJson(), equals(expected));
     }
     // Invalid for this case
-    expect(() => parseJson(SOURCE_MAP_BUNDLE), throws);
+    expect(() => parseJson(SOURCE_MAP_BUNDLE as dynamic), throws);
 
     var mapping = parseJsonExtended(SOURCE_MAP_BUNDLE);
     expect(mapping.toJson(), equals(SOURCE_MAP_BUNDLE));