Introduce getPrefixEndToken to avoid too big context in messages.

pkg/compiler/lib/src/elements/modelx.dart

Index: pkg/compiler/lib/src/elements/modelx.dart
diff --git a/pkg/compiler/lib/src/elements/modelx.dart b/pkg/compiler/lib/src/elements/modelx.dart
index 3f0d70ccab257c5f6f3694efbdd73a1346b78409..121a88214fe2fb52ce9177beeffbb362505724de 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -2115,14 +2115,7 @@ abstract class FunctionElementX extends BaseFunctionElementX
     if (span != null && hasNode) {
       FunctionExpression functionExpression = node.asFunctionExpression();
       if (functionExpression != null) {
-        Token begin = functionExpression.getBeginToken();
-        Token end;
-        if (functionExpression.parameters != null) {
-          end = functionExpression.parameters.getEndToken();
-        } else {
-          end = functionExpression.name.getEndToken();
-        }
-        span = new SourceSpan.fromTokens(span.uri, begin, end);
+        span = new SourceSpan.fromNode(span.uri, functionExpression);
       }
     }
     return span;