New parser event needed to check case expressions.

R=paulberry@google.com

Review-Url: https://codereview.chromium.org/2900993002 .

pkg/front_end/lib/src/fasta/parser/listener.dart

@@ -65,6 +65,12 @@ class Listener {
     logEvent("Cascade");
   }
 
+  void beginCaseExpression(Token caseKeyword) {}
+
+  void endCaseExpression(Token colon) {
+    logEvent("CaseExpression");
+  }
+
   void beginClassBody(Token token) {}
 
   /// Handle the end of the body of a class declaration.  The only substructures

pkg/front_end/lib/src/fasta/parser/parser.dart

@@ -3722,7 +3722,9 @@ class Parser {
           labelCount++;
         }
         Token caseKeyword = token;
+        listener.beginCaseExpression(token);
         token = parseExpression(token.next);
+        listener.endCaseExpression(token);
         Token colonToken = token;
         token = expect(':', token);
         listener.handleCaseMatch(caseKeyword, colonToken);