Implement the new '??=' operator in analyzer.

pkg/analyzer/lib/src/generated/static_type_analyzer.dart

Index: pkg/analyzer/lib/src/generated/static_type_analyzer.dart
diff --git a/pkg/analyzer/lib/src/generated/static_type_analyzer.dart b/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
index d1997f3f0495f933b7d00c8740fd4edde86a7c5e..a78311fcd86a8d81b2602191ac34b6c0ddfc877d 100644
--- a/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
+++ b/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
@@ -157,6 +157,11 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
         overrideType = propagatedType;
       }
       _resolver.overrideExpression(node.leftHandSide, overrideType, true);
+    } else if (operator == sc.TokenType.QUESTION_QUESTION_EQ) {
+      // The static type of a compound assignment using ??= is the least upper
+      // bound of the static types of the LHS and RHS.
+      _analyzeLeastUpperBound(node, node.leftHandSide, node.rightHandSide);
+      return null;
     } else {
       ExecutableElement staticMethodElement = node.staticElement;
       DartType staticType = _computeStaticReturnType(staticMethodElement);