Fix for parsing default parameters.

pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart

Index: pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart b/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
index 175ce815a37f718341692f394f63bfc4beff0463..af1af4308680841c753f1e720649e1414569d292 100644
--- a/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
+++ b/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
@@ -637,12 +637,16 @@ class AstBuilder extends ScopeListener {
       }
     }
 
-    if (defaultValue != null) {
-      node = ast.defaultFormalParameter(node, _toAnalyzerParameterKind(kind),
-          toAnalyzerToken(defaultValue.separator), defaultValue.value);
+    ParameterKind analyzerKind = _toAnalyzerParameterKind(kind);
+    if (analyzerKind != ParameterKind.REQUIRED) {
+      node = ast.defaultFormalParameter(node, analyzerKind,
+          toAnalyzerToken(defaultValue?.separator), defaultValue?.value);
     }
 
-    scope[name.name] = name.staticElement = new AnalyzerParameterElement(node);
+    if (name != null) {
+      scope[name.name] =
+          name.staticElement = new AnalyzerParameterElement(node);
+    }
     push(node);
   }