Adds check for `AsExpression`s (Fix #145).

lib/src/rules/avoid_as.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 linter.src.rules.avoid_as;
+
+import 'package:analyzer/src/generated/ast.dart'
+    show AsExpression, AstVisitor, SimpleAstVisitor;
+import 'package:linter/src/linter.dart';
+
+const desc = r'Avoid using `as`.';
+
+const details = r'''
+From the [flutter style guide]
+(https://github.com/flutter/engine/blob/master/sky/specs/style-guide.md):
+
+**AVOID** using `as`.
+
+If you know the type is correct, use an assertion or assign to a more
+narrowly-typed variable (this avoids the type check in release mode; `as`
+is not compiled out in release mode). If you don't know whether the type is
+correct, check using `is` (this avoids the exception that `as` raises).
+
+**BAD:**
+```
+try {
+   (pm as Person).firstName = 'Seth';
+} on CastError { }
+```
+
+**GOOD:**
+```
+Person person = pm;
+person.firstName = 'Seth';
+```
+''';
+
+class AvoidAs extends LintRule {
+  AvoidAs()
+      : super(
+            name: 'avoid_as',
+            description: desc,
+            details: details,
+            group: Group.style);
+
+  @override
+  AstVisitor getVisitor() => new Visitor(this);
+}
+
+class Visitor extends SimpleAstVisitor {
+  final LintRule rule;
+  Visitor(this.rule);
+
+  @override
+  visitAsExpression(AsExpression node) {
+    rule.reportLint(node);
+  }
+}