[VM/nnbd] Field initializers may allocate a context storing 'this' when needed.

This fixes these co19 tests in strong mode:
LanguageFeatures/nnbd/late_A05_t01
LanguageFeatures/nnbd/late_A05_t02

Change-Id: I90e9f91eec120d82992e835f79396c2a7770fcbc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/144814
Commit-Queue: Régis Crelier <[email protected]>
Reviewed-by: Alexander Markov <[email protected]>

Author: crelier

runtime/vm/compiler/frontend/kernel_binary_flowgraph.cc

@@ -54,6 +54,7 @@ FlowGraph* StreamingFlowGraphBuilder::BuildGraphOfFieldInitializer() {
     body += Constant(
         Instance::ZoneHandle(Z, constant_reader_.ReadConstantExpression()));
   } else {
+    body += SetupCapturedParameters(parsed_function()->function());
     body += BuildExpression();  // read initializer.
   }
   body += Return(TokenPosition::kNoSource);

runtime/vm/object.h

@@ -2779,7 +2779,7 @@ class Function : public Object {
 
   bool HasThisParameter() const {
     return IsDynamicFunction(/*allow_abstract=*/true) ||
-           IsGenerativeConstructor();
+           IsGenerativeConstructor() || (IsFieldInitializer() && !is_static());
   }
 
   bool IsDynamicFunction(bool allow_abstract = false) const {
@@ -3245,7 +3245,7 @@ class Function : public Object {
     return kind() == RawFunction::kImplicitSetter;
   }
 
-  // Returns true if this function represents an the initializer for a static or
+  // Returns true if this function represents an initializer for a static or
   // instance field. The function returns the initial value and the caller is
   // responsible for setting the field.
   bool IsFieldInitializer() const {