[generators] Add some explanation on forcing context allocation.

src/full-codegen/mips/full-codegen-mips.cc

Index: src/full-codegen/mips/full-codegen-mips.cc
diff --git a/src/full-codegen/mips/full-codegen-mips.cc b/src/full-codegen/mips/full-codegen-mips.cc
index 8a093ab33e243eb877e0b7b6e2ec951c381e708e..2ea2edff9cfffed531e4a2681ee98ce6ab1dd848 100644
--- a/src/full-codegen/mips/full-codegen-mips.cc
+++ b/src/full-codegen/mips/full-codegen-mips.cc
@@ -1903,7 +1903,10 @@ void FullCodeGenerator::EmitGeneratorResume(Expression *generator,
   __ lw(a2, FieldMemOperand(a1, JSGeneratorObject::kReceiverOffset));
   __ push(a2);
 
-  // Push holes for the rest of the arguments to the generator function.
+  // Push holes for arguments to generator function. Since the parser forced
+  // context allocation for any variables in generators, the actual argument
+  // values have already been copied into the context and these dummy values
+  // will never be used.
   __ lw(a3, FieldMemOperand(t0, JSFunction::kSharedFunctionInfoOffset));
   __ lw(a3,
         FieldMemOperand(a3, SharedFunctionInfo::kFormalParameterCountOffset));