First step towards having patch files for generic libraries.

lib/compiler/implementation/source_file.dart

Index: lib/compiler/implementation/source_file.dart
diff --git a/lib/compiler/implementation/source_file.dart b/lib/compiler/implementation/source_file.dart
index 276b0fe5d2af924297cf49f770fdcf8a5e6e75c5..222420c457e2b7343000e4964886f3cb20c8626d 100644
--- a/lib/compiler/implementation/source_file.dart
+++ b/lib/compiler/implementation/source_file.dart
@@ -42,7 +42,8 @@ class SourceFile {
     for (int i=0; i < starts.length; i++) {
       if (starts[i] > position) return i-1;
     }
-    throw 'bad position';
+    throw 'bad position #$position in file $filename with '
+          'length ${text.length}.';
   }
 
   int getColumn(int line, int position) {