Introduce getPrefixEndToken to avoid too big context in messages.

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 4b37788a616c93a2e1671d05a55dca05c3f41390..e6b4f4c92b8bef96248f21eff940140ba9ab560c 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -1795,6 +1795,18 @@ class _CompilerDiagnosticReporter extends DiagnosticReporter {
       uri = currentElement.compilationUnit.script.resourceUri;
       assert(invariant(currentElement, () {
 
+        bool sameToken(Token token, Token sought) {
+          if (token == sought) return true;
+          if (token.stringValue == '>>' ||
+              token.stringValue == '>>>') {
+            // `>>` and `>>>` are converted to `>` in the parser when needed.
+            return sought.stringValue == '>' &&
+                token.charOffset <= sought.charOffset &&
+                sought.charOffset < token.charEnd;
+          }
+          return false;
+        }
+
         /// Check that [begin] and [end] can be found between [from] and [to].
         validateToken(Token from, Token to) {
           if (from == null || to == null) return true;
@@ -1802,10 +1814,10 @@ class _CompilerDiagnosticReporter extends DiagnosticReporter {
           bool foundEnd = false;
           Token token = from;
           while (true) {
-            if (token == begin) {
+            if (sameToken(token, begin)) {
               foundBegin = true;
             }
-            if (token == end) {
+            if (sameToken(token, end)) {
               foundEnd = true;
             }
             if (foundBegin && foundEnd) {
@@ -1859,7 +1871,7 @@ class _CompilerDiagnosticReporter extends DiagnosticReporter {
   }
 
   SourceSpan spanFromNode(Node node) {
-    return spanFromTokens(node.getBeginToken(), node.getEndToken());
+    return spanFromTokens(node.getBeginToken(), node.getPrefixEndToken());
   }
 
   SourceSpan spanFromElement(Element element) {