Add API to force the computation of constant values

pkg/analyzer/lib/dart/element/element.dart

 // Copyright (c) 2014, 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.
 
 /**
  * Defines the element model. The element model describes the semantic (as
  * opposed to syntactic) structure of Dart code. The syntactic structure of the
  * code is modeled by the [AST structure](../ast/ast.dart).
  *
  * The element model consists of two closely related kinds of objects: elements
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   /**
    * Return a representation of the value of this annotation.
    *
    * Return `null` if the value of this annotation could not be computed because
    * of errors.
    */
   DartObject get constantValue;
 
   /**
+   * Return a representation of the value of this annotation, forcing the value
+   * to be computed if it had not previously been computed, or `null` if the
+   * value of this annotation could not be computed because of errors.
+   */
+  DartObject computeConstantValue();
+
+  /**
    * Return the element representing the field, variable, or const constructor
    * being used as an annotation.
    */
   Element get element;
 
   /**
    * Return `true` if this annotation marks the associated element as being
    * deprecated.
    */
   bool get isDeprecated;
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   /**
    * Return a representation of the value of this variable.
    *
    * Return `null` if either this variable was not declared with the 'const'
    * modifier or if the value of this variable could not be computed because of
    * errors.
    */
   DartObject get constantValue;
 
   /**
+   * Return a representation of the value of this variable, forcing the value
+   * to be computed if it had not previously been computed, or `null` if either
+   * this variable was not declared with the 'const' modifier or if the value of
+   * this variable could not be computed because of errors.
+   */
+  DartObject computeConstantValue();
+
+  /**
    * Return `true` if this variable element did not have an explicit type
    * specified for it.
    */
   bool get hasImplicitType;
 
   /**
    * Return a synthetic function representing this variable's initializer, or
    * `null` if this variable does not have an initializer. The function will
    * have no parameters. The return type of the function will be the
    * compile-time type of the initialization expression.
@@ skipping 51 matching lines @@
    */
   bool get isStatic;
 
   /**
    * Return the declared type of this variable, or `null` if the variable did
    * not have a declared type (such as if it was declared using the keyword
    * 'var').
    */
   DartType get type;
 }