Implements simple statement executer
Implements simple statement executer

This change adds support for executing:
 - BlockStatement
 - ExpressionStatement
 - IfStatement
 - VariableDeclaration

Currently only BlockStatement introduces a new environment and
VariableDeclaration modifies the environment.

BUG=
R=kmillikin@google.com

Review-Url: https://codereview.chromium.org/2775293002 .

dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk