[vm/testing] Make vm/dart/gc/scavenger_abort_2_test more robust

The test tries to keep fields alive by my making a

  toString() => field1 + field2 + ...

Though some field values don't have a `+` operator (e.g. instances of `class C`).

Improving TFA precision can infer that some of these `+` operations will throw,
which then makes other `+` operations unreachable, which removes usages
of those other fields, which will tree shake those other fields, which
will make the test not test anymore what it's intended to.

=> Make the test more robust by using string concatenation instead.

Change-Id: I80788af516083ea3d78ad910eb394e4b7e122384
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/507000
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

runtime/tests/vm/dart/gc/scavenger_abort_2_test.dart

@@ -26,19 +26,8 @@ class A {
 
   // Prevent fields from being optimized away as write-only.
   String toString() {
-    return field1 +
-        field2 +
-        field3 +
-        field4 +
-        field5 +
-        field6 +
-        field7 +
-        field8 +
-        field9 +
-        field10 +
-        field11 +
-        field12 +
-        field13;
+    return '$field1$field2$field3$field4$field5$field6$field7'
+        '$field8$field9$field10$field11$field12$field13';
   }
 }
 
@@ -64,23 +53,9 @@ class B {
 
   // Prevent fields from being optimized away as write-only.
   String toString() {
-    return field1 +
-        field2 +
-        field3 +
-        field4 +
-        field5 +
-        field6 +
-        field7 +
-        field8 +
-        field9 +
-        field10 +
-        field11 +
-        field12 +
-        field13 +
-        field14 +
-        field15 +
-        field16 +
-        field17;
+    return '$field1$field2$field3$field4$field5$field6$field7'
+        '$field8$field9$field10$field11$field12$field13$field14'
+        '$field15$field16$field17';
   }
 }
 
@@ -108,25 +83,9 @@ class C {
 
   // Prevent fields from being optimized away as write-only.
   String toString() {
-    return field1 +
-        field2 +
-        field3 +
-        field4 +
-        field5 +
-        field6 +
-        field7 +
-        field8 +
-        field9 +
-        field10 +
-        field11 +
-        field12 +
-        field13 +
-        field14 +
-        field15 +
-        field16 +
-        field17 +
-        field18 +
-        field19;
+    return '$field1$field2$field3$field4$field5$field6$field7'
+        '$field8$field9$field10$field11$field12$field13$field14'
+        '$field15$field16$field17$field18$field19';
   }
 }
 
@@ -166,37 +125,10 @@ class Old {
 
   // Prevent fields from being optimized away as write-only.
   String toString() {
-    return new1 +
-        new2 +
-        new3 +
-        new4 +
-        new5 +
-        new6 +
-        new7 +
-        new8 +
-        new9 +
-        new10 +
-        new11 +
-        new12 +
-        new13 +
-        new14 +
-        new15 +
-        new16 +
-        new17 +
-        new18 +
-        new19 +
-        new20 +
-        new21 +
-        new22 +
-        new23 +
-        new24 +
-        new25 +
-        new26 +
-        new27 +
-        new28 +
-        new29 +
-        new30 +
-        new31;
+    return '$new1$new2$new3$new4$new5$new6$new7$new8$new9$new10'
+        '$new11$new12$new13$new14$new15$new16$new17$new18$new19$new20'
+        '$new21$new22$new23$new24$new25$new26$new27$new28$new29$new30'
+        '$new31';
   }
 }