Add API to force the computation of constant values

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

Index: pkg/analyzer/lib/dart/element/element.dart
diff --git a/pkg/analyzer/lib/dart/element/element.dart b/pkg/analyzer/lib/dart/element/element.dart
index 1a7345767d2ad35c013d78d41de2733d0f4be015..e0a91d93d03bf5e1b094317c178969e1ef2841b7 100644
--- a/pkg/analyzer/lib/dart/element/element.dart
+++ b/pkg/analyzer/lib/dart/element/element.dart
@@ -806,6 +806,13 @@ abstract class ElementAnnotation implements ConstantEvaluationTarget {
   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.
    */
@@ -1931,6 +1938,14 @@ abstract class VariableElement implements Element, ConstantEvaluationTarget {
   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.
    */