[vm/backend] Add missing compiler::target:: prefix.

This omission caused dartkp-strong-linux-release-simarm-crossword:language/type_variable/typedef_test to fail in pending CL https://dart-review.googlesource.com/c/sdk/+/166920/19

TEST=patchset flutter#20 of above mentioned CL.

Change-Id: Iae11ccdbb1335bfcf1596636543cef4928f114b3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/176361
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Régis Crelier <regis@google.com>

Author: crelier

runtime/vm/compiler/backend/flow_graph_compiler.cc

@@ -2675,8 +2675,9 @@ SubtypeTestCachePtr FlowGraphCompiler::GenerateUninstantiatedTypeTest(
     // Check if type arguments are null, i.e. equivalent to vector of dynamic.
     __ CompareObject(kTypeArgumentsReg, Object::null_object());
     __ BranchIf(EQUAL, is_instance_lbl);
-    __ LoadFieldFromOffset(kScratchReg, kTypeArgumentsReg,
-                           TypeArguments::type_at_offset(type_param.index()));
+    __ LoadFieldFromOffset(
+        kScratchReg, kTypeArgumentsReg,
+        compiler::target::TypeArguments::type_at_offset(type_param.index()));
     // kScratchReg: Concrete type of type.
     // Check if type argument is dynamic, Object?, or void.
     __ CompareObject(kScratchReg, Object::dynamic_type());