Migration: create flow anaylsis before visiting parameters.

This is necessary in case any parameters are found that trigger flow
analysis methods to be called.

Should address ~24 exceptions whose stack trace contains the line:

EdgeBuilder.visitBooleanLiteral (package:nnbd_migration/src/edge_builder.dart:390:19)

Change-Id: I08dc31a8acdaaf0685516aab872905f620602806
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/116488
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/nnbd_migration/lib/src/edge_builder.dart

@@ -1647,9 +1647,9 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
     assert(_currentFunctionType == null);
     metadata.accept(this);
     returnType?.accept(this);
+    _createFlowAnalysis(body);
     parameters?.accept(this);
     _currentFunctionType = _variables.decoratedElementType(declaredElement);
-    _createFlowAnalysis(body);
     _addParametersToFlowAnalysis(parameters);
     // Push a scope of post-dominated declarations on the stack.
     _postDominatedLocals.pushScope(elements: declaredElement.parameters);

pkg/nnbd_migration/test/edge_builder_test.dart

@@ -1439,6 +1439,15 @@ int f(bool b, int i) {
     assertLUB(nullable_conditional, nullable_i, always);
   }
 
+  test_constructor_default_parameter_value_bool() async {
+    await analyze('''
+class C {
+  C([bool b = true]);
+}
+''');
+    assertNoUpstreamNullability(decoratedTypeAnnotation('bool b').node);
+  }
+
   test_constructor_named() async {
     await analyze('''
 class C {