ASAN build failure after "[ VM ] Add support for sampling old space allocations"

[sstrickl]

The build is failing on the ASAN bots due to a memory leak in gen_snapshot:

Example log:

...
[2370/2577] AR obj/runtime/libdart_precompiler_testing.a
[2371/2577] ACTION //runtime/bin:generate_snapshot_bin(//build/toolchain/linux:clang_x64)
FAILED: gen/runtime/bin/vm_snapshot_data.bin gen/runtime/bin/vm_snapshot_instructions.bin gen/runtime/bin/isolate_snapshot_data.bin gen/runtime/bin/isolate_snapshot_instructions.bin 
python3 ../../build/gn_run_binary.py compiled_action gen_snapshot --sound-null-safety --deterministic --snapshot_kind=core --vm_snapshot_data=gen/runtime/bin/vm_snapshot_data.bin --vm_snapshot_instructions=gen/runtime/bin/vm_snapshot_instructions.bin --isolate_snapshot_data=gen/runtime/bin/isolate_snapshot_data.bin --isolate_snapshot_instructions=gen/runtime/bin/isolate_snapshot_instructions.bin /b/s/w/ir/cache/builder/sdk/out/ReleaseASANX64/vm_platform_strong_stripped.dill
Command failed: /b/s/w/ir/cache/builder/sdk/out/ReleaseASANX64/gen_snapshot --sound-null-safety --deterministic --snapshot_kind=core --vm_snapshot_data=gen/runtime/bin/vm_snapshot_data.bin --vm_snapshot_instructions=gen/runtime/bin/vm_snapshot_instructions.bin --isolate_snapshot_data=gen/runtime/bin/isolate_snapshot_data.bin --isolate_snapshot_instructions=gen/runtime/bin/isolate_snapshot_instructions.bin /b/s/w/ir/cache/builder/sdk/out/ReleaseASANX64/vm_platform_strong_stripped.dill
output: 
=================================================================
==7105==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 1258 byte(s) in 95 object(s) allocated from:
    #0 0x55d24a27986e in malloc ../staging/llvm_build/tools/clang/stage2-bins/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/compiler-rt/lib/asan/asan_malloc_linux.cpp:69:3
    #1 0x55d24a32176e in dart::malloc(unsigned long) ../../out/ReleaseASANX64/../../runtime/platform/allocation.cc:20:18
    #2 0x55d24a6d460f in dart::String::ToMallocCString() const ../../out/ReleaseASANX64/../../runtime/vm/object.cc:24460:48
    #3 0x55d24a6d460f in dart::Class::SetUserVisibleNameInClassTable() ../../out/ReleaseASANX64/../../runtime/vm/object.cc:5161:60
    #4 0x55d24a48023e in dart::ClassFinalizer::ProcessPendingClasses() ../../out/ReleaseASANX64/../../runtime/vm/class_finalizer.cc:219:11
    #5 0x55d24a3750d9 in dart::Finish(dart::Thread*) ../../out/ReleaseASANX64/../../runtime/vm/bootstrap.cc:44:8
    #6 0x55d24a3750d9 in dart::BootstrapFromKernel(dart::Thread*, unsigned char const*, long) ../../out/ReleaseASANX64/../../runtime/vm/bootstrap.cc:144:5
    #7 0x55d24a37455c in dart::Bootstrap::DoBootstrapping(unsigned char const*, long) ../../out/ReleaseASANX64/../../runtime/vm/bootstrap.cc:192:10
    #8 0x55d24a689aa3 in dart::Object::Init(dart::IsolateGroup*, unsigned char const*, long) ../../out/ReleaseASANX64/../../runtime/vm/object.cc:2477:15
    #9 0x55d24a4bbde1 in dart::Dart::InitIsolateFromSnapshot(dart::Thread*, dart::Isolate*, unsigned char const*, unsigned char const*, unsigned char const*, long) ../../out/ReleaseASANX64/../../runtime/vm/dart.cc:853:11
    #10 0x55d24a4bca1a in dart::Dart::InitializeIsolate(unsigned char const*, unsigned char const*, unsigned char const*, long, dart::IsolateGroup*, void*) ../../out/ReleaseASANX64/../../runtime/vm/dart.cc:990:9
    #11 0x55d24b95e5e8 in dart::CreateIsolate(dart::IsolateGroup*, bool, char const*, void*, char**) ../../out/ReleaseASANX64/../../runtime/vm/dart_api_impl.cc:1301:12
    #12 0x55d24b95f2a2 in Dart_CreateIsolateGroupFromKernel ../../out/ReleaseASANX64/../../runtime/vm/dart_api_impl.cc:1431:26
    #13 0x55d24a2b2e55 in dart::bin::CreateIsolateAndSnapshot(dart::bin::CommandLineOptions const&) ../../out/ReleaseASANX64/../../runtime/bin/gen_snapshot.cc:756:15
    #14 0x55d24a2b2e55 in dart::bin::main(int, char**) ../../out/ReleaseASANX64/../../runtime/bin/gen_snapshot.cc:940:16
    #15 0x7f68e405bc86 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21c86) (BuildId: f7307432a8b162377e77a182b6cc2e53d771ec4b)

SUMMARY: AddressSanitizer: 1258 byte(s) leaked in 95 allocation(s).

ninja: build stopped: subcommand failed.
buildtools/ninja/ninja -C out/ReleaseASANX64 -j200 -l64 runtime done.

/cc @bkonyi