update fasta scanner to always generate INDEX or INDEX_EQ tokens

pkg/compiler/lib/src/tree/unparser.dart

Index: pkg/compiler/lib/src/tree/unparser.dart
diff --git a/pkg/compiler/lib/src/tree/unparser.dart b/pkg/compiler/lib/src/tree/unparser.dart
index e75d7bc78f348ee0d8e2153d369345f43aa87d18..b98960dcbe055c32609dca896c43408e295f02bd 100644
--- a/pkg/compiler/lib/src/tree/unparser.dart
+++ b/pkg/compiler/lib/src/tree/unparser.dart
@@ -386,7 +386,11 @@ class Unparser extends Indentation implements Visitor {
     if (node.nodes != null) {
       unparseNodeListFrom(node, node.nodes, spaces: spaces);
     }
-    if (node.endToken != null) write(node.endToken.lexeme);
+    // if the NodeList is a single "[]" token

[ahe, 2017/06/01 14:45:01]

if -> If

+    // then beginToken == endToken and only write beginToken.
+    if (node.endToken != null && node.endToken != node.beginToken) {
+      write(node.endToken.lexeme);
+    }
   }
 
   visitNodeList(NodeList node) {