[vm] Add an isolate group flag for branch coverage.

Branch coverage can't be enabled by a simple global flag, because this
causes problems when functions in an app-jit snapshot that didn't have
the flag enabled are compiled by a VM that has the flag enabled. In
particular, the coverage array will see a different set of token
positions, causing some important asserts to fail.

Change-Id: I35227252b5d271f20b01de99466c06708ec83ed8
TEST=CI
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/223360
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Liam Appelbe <liama@google.com>

runtime/bin/dartdev_isolate.cc

@@ -210,6 +210,7 @@ void DartDevIsolate::DartDevRunner::RunCallback(uword args) {
   flags.use_field_guards = true;
   flags.use_osr = true;
   flags.is_system_isolate = true;
+  flags.branch_coverage = false;
 
   char* error;
   Dart_Isolate dartdev_isolate = runner->create_isolate_(

runtime/include/dart_api.h

@@ -606,6 +606,7 @@ typedef struct {
   bool null_safety;
   bool is_system_isolate;
   bool snapshot_is_dontneed_safe;
+  bool branch_coverage;
 } Dart_IsolateFlags;
 
 /**

runtime/vm/dart.cc

@@ -1173,6 +1173,8 @@ char* Dart::FeaturesString(IsolateGroup* isolate_group,
       ADD_ISOLATE_GROUP_FLAG(use_field_guards, use_field_guards,
                              FLAG_use_field_guards);
       ADD_ISOLATE_GROUP_FLAG(use_osr, use_osr, FLAG_use_osr);
+      ADD_ISOLATE_GROUP_FLAG(branch_coverage, branch_coverage,
+                             FLAG_branch_coverage);
     }
 
 // Generated code must match the host architecture and ABI.

runtime/vm/flag_list.h

@@ -246,6 +246,7 @@ constexpr bool FLAG_support_il_printer = false;
   D(support_rr, bool, false, "Support running within RR.")                     \
   P(verify_entry_points, bool, false,                                          \
     "Throw API error on invalid member access throuh native API. See "         \
-    "entry_point_pragma.md")
+    "entry_point_pragma.md")                                                   \
+  C(branch_coverage, false, false, bool, false, "Enable branch coverage")
 
 #endif  // RUNTIME_VM_FLAG_LIST_H_

runtime/vm/isolate.h

@@ -163,7 +163,9 @@ typedef FixedCache<intptr_t, CatchEntryMovesRefPtr, 16> CatchEntryMovesCache;
     load_vmservice_library, false)                                             \
   V(NONPRODUCT, use_osr, UseOsr, use_osr, FLAG_use_osr)                        \
   V(NONPRODUCT, snapshot_is_dontneed_safe, SnapshotIsDontNeedSafe,             \
-    snapshot_is_dontneed_safe, false)
+    snapshot_is_dontneed_safe, false)                                          \
+  V(NONPRODUCT, branch_coverage, BranchCoverage, branch_coverage,              \
+    FLAG_branch_coverage)
 
 #define BOOL_ISOLATE_FLAG_LIST_DEFAULT_GETTER(V)                               \
   V(PRODUCT, copy_parent_code, CopyParentCode, copy_parent_code, false)        \
@@ -796,7 +798,8 @@ class IsolateGroup : public IntrusiveDListEntry<IsolateGroup> {
   V(Obfuscate)                                                                 \
   V(UseFieldGuards)                                                            \
   V(UseOsr)                                                                    \
-  V(SnapshotIsDontNeedSafe)
+  V(SnapshotIsDontNeedSafe)                                                    \
+  V(BranchCoverage)
 
   // Isolate group specific flags.
   enum FlagBits {