pushScope(tree) {
    let scope = this.scopeBuilder_.getScopeForTree(tree);
    if (!scope) throw new Error('BlockBindingTransformer tree with no scope');
    if (this.scope_) this.scope_.blockBindingRenames = this.blockRenames_;
    this.scope_ = scope;
    this.blockRenames_ = [];
    return scope;
  }
Pushes new scope
@param {Scope} scope
@return {Scope}