Don't double-escape in strings in the VM Service, and don't use \u0000 to determine the string lengt

runtime/vm/report_test.cc

Index: runtime/vm/report_test.cc
diff --git a/runtime/vm/report_test.cc b/runtime/vm/report_test.cc
index e10bd5b32c071d2ac8864636c3b6fb0abcc7c050..b17905541a3a8c4941e49aacf98ff0fadf6ecd82 100644
--- a/runtime/vm/report_test.cc
+++ b/runtime/vm/report_test.cc
@@ -36,7 +36,7 @@ TEST_CASE(TraceJSWarning) {
                        "\"message\":{\"type\":\"@String\"",
                        js.ToCString());
       // Skip private _OneByteString.
-      EXPECT_SUBSTRING("\"valueAsString\":\"\\\"High Voltage\\\"\"}}",
+      EXPECT_SUBSTRING("\"valueAsString\":\"High Voltage\"",
                        js.ToCString());
     }
   }
@@ -53,7 +53,7 @@ TEST_CASE(TraceJSWarning) {
                    "\"message\":{\"type\":\"@String\"",
                    trace_buffer->At(0)->message);
   // Skip private _OneByteString.
-  EXPECT_SUBSTRING("\"valueAsString\":\"\\\"High Voltage\\\"\"}}",
+  EXPECT_SUBSTRING("\"valueAsString\":\"High Voltage\"",
                    trace_buffer->At(0)->message);
 
   EXPECT_SUBSTRING("{\"type\":\"JSCompatibilityWarning\",\"script\":{\"type\":"
@@ -63,7 +63,7 @@ TEST_CASE(TraceJSWarning) {
                    "\"message\":{\"type\":\"@String\"",
                    trace_buffer->At(1)->message);
   // Skip private _OneByteString.
-  EXPECT_SUBSTRING("\"valueAsString\":\"\\\"Low Voltage\\\"\"}}",
+  EXPECT_SUBSTRING("\"valueAsString\":\"Low Voltage\"",
                    trace_buffer->At(1)->message);
 
   delete trace_buffer;