Create and test source mapping for invocations.

pkg/compiler/lib/src/io/line_column_provider.dart

Index: pkg/compiler/lib/src/io/line_column_provider.dart
diff --git a/pkg/compiler/lib/src/io/line_column_provider.dart b/pkg/compiler/lib/src/io/line_column_provider.dart
index 4d17583fb9f5b972cf93b2dbf67c7811e4b4faa9..590e4e2a8215006494c9997753fbc81611a5ebe5 100644
--- a/pkg/compiler/lib/src/io/line_column_provider.dart
+++ b/pkg/compiler/lib/src/io/line_column_provider.dart
@@ -13,6 +13,9 @@ abstract class LineColumnProvider {
 
   /// Returns the column number (0-based) for [offset] at the given [line].
   int getColumn(int line, int offset);
+
+  /// Returns the offset for 0-based [line] and [column] numbers.
+  int getOffset(int line, int column);
 }
 
 /// [CodeOutputListener] that collects line information.
@@ -65,6 +68,8 @@ class LineColumnCollector extends CodeOutputListener
     return offset - lineStarts[line];
   }
 
+  int getOffset(int line, int column) => lineStarts[line] + column;
+
   @override
   void onDone(int length) {
     lineStarts.add(length + 1);