vm/service/observatory: Add coverage for functions, some tests, and fix buggy filter

runtime/vm/coverage.cc

Index: runtime/vm/coverage.cc
diff --git a/runtime/vm/coverage.cc b/runtime/vm/coverage.cc
index 2eb0d5edbd24b961da726515c393b7fda196381b..37d38ba5d76936fcd6de8bed6bbfd2062a73f7f6 100644
--- a/runtime/vm/coverage.cc
+++ b/runtime/vm/coverage.cc
@@ -175,6 +175,10 @@ void CodeCoverage::PrintClass(const Library& lib,
         pos_to_line.Clear();
         break;
       }
+      if (!filter->ShouldOutputCoverageFor(lib, saved_url, cls, function)) {
+        i++;
+        continue;
+      }
       CompileAndAdd(function, hits_arr, pos_to_line);
       if (function.HasImplicitClosureFunction()) {
         function = function.ImplicitClosureFunction();