Fix for alphanumerics following braces.

lib/src/rules/unnecessary_brace_in_string_interp.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library unnecessary_brace_in_string_interp;
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/error.dart';
+import 'package:analyzer/src/generated/scanner.dart';
 import 'package:analyzer/src/services/lint.dart';
 
-const msg = '''
-Interpolated simple identifiers (not followed by an alphanumeric string) do
-not need braces.
-''';
+final RegExp alphaNumeric = new RegExp(r'^[a-zA-Z0-9]');
+
+const msg =
+    '''Interpolated simple identifiers (not followed by an alphanumeric string) do
+not need braces.''';
 
 const name = 'UnnecessaryBraceInStringInterp';
 
+bool isAlphaNumeric(Token token) =>
+    token is StringToken && token.lexeme.startsWith(alphaNumeric);
+
 class UnnecessaryBraceInStringInterp extends Linter {
   @override
   AstVisitor getVisitor() => new Visitor(reporter);
 }
 
-class Visitor extends SimpleAstVisitor<Object> {
+class Visitor extends SimpleAstVisitor {
   ErrorReporter reporter;
   Visitor(this.reporter);
 
   @override
   visitStringInterpolation(StringInterpolation node) {
     var expressions = node.elements.where((e) => e is InterpolationExpression);
     for (InterpolationExpression expression in expressions) {
-      if (expression.expression is SimpleIdentifier &&
-          expression.rightBracket != null) {
-        reporter.reportErrorForNode(new LintCode(name, msg), expression, []);
+      if (expression.expression is SimpleIdentifier) {
+        Token bracket = expression.rightBracket;
+        if (bracket != null && !isAlphaNumeric(bracket.next)) {
+          reporter.reportErrorForNode(new LintCode(name, msg), expression, []);
+        }
       }
     }
   }
 }