Recover correctly from this-parameter on non-constructors.

pkg/front_end/lib/src/fasta/builder/procedure_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/procedure_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/procedure_builder.dart b/pkg/front_end/lib/src/fasta/builder/procedure_builder.dart
index 64f19a721a93039d7c0ee2a01b357d7756c751e9..fc62cee4ee4ebc77e16a3214bd25ce04cb0b2a71 100644
--- a/pkg/front_end/lib/src/fasta/builder/procedure_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/procedure_builder.dart
@@ -69,7 +69,7 @@ abstract class ProcedureBuilder<T extends TypeBuilder> extends MemberBuilder {
     if (formals == null) return parent;
     Map<String, Builder> local = <String, Builder>{};
     for (FormalParameterBuilder formal in formals) {
-      if (!formal.hasThis) {
+      if (!isConstructor || !formal.hasThis) {
         local[formal.name] = formal;
       }
     }