519c2221 triggered ASAN errors

[ghost]

Several ASAN issues have surfaced after 519c222:
https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8833423770193018001/+/u/test_results/new_test_failures__logs_

vm/cc/DartAPI_InvokeVMServiceMethod broke (Pass -> Crash, expected Pass)
vm/cc/DartAPI_InvokeVMServiceMethod_Loop broke (Pass -> Crash, expected Pass)
vm/dart_2/isolates/reload_active_stack_test broke (Pass -> RuntimeError, expected Pass)
vm/dart_2/isolates/reload_many_isolates_test broke (Pass -> RuntimeError, expected Pass)
vm/dart_2/isolates/reload_no_active_stack_test broke (Pass -> RuntimeError, expected Pass)

E.g.:

=================================================================
==22980==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 256 byte(s) in 1 object(s) allocated from:
    #0 0x5590ae7a1153 in realloc ../staging/llvm_build/tools/clang/stage2-bins/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/compiler-rt/lib/asan/asan_malloc_linux.cpp:148:3
    #1 0x5590af231206 in dart::realloc(void*, unsigned long) ../../out/ReleaseASANX64/../../runtime/platform/allocation.cc:20:18
    #2 0x5590af23473e in dart::TextBuffer::EnsureCapacity(long) ../../out/ReleaseASANX64/../../runtime/platform/text_buffer.cc:146:45
    #3 0x5590af232f1c in dart::BaseTextBuffer::VPrintf(char const*, __va_list_tag*) ../../out/ReleaseASANX64/../../runtime/platform/text_buffer.cc:30:10
    #4 0x5590af232f1c in dart::BaseTextBuffer::Printf(char const*, ...) ../../out/ReleaseASANX64/../../runtime/platform/text_buffer.cc:17:18
    #5 0x5590af371a06 in dart::Dart::FeaturesString(dart::IsolateGroup*, bool, dart::Snapshot::Kind) ../../out/ReleaseASANX64/../../runtime/vm/dart.cc:1098:5
    #6 0x5590af82bbe4 in dart::Service::PrintJSONForVM(dart::JSONStream*, bool) ../../out/ReleaseASANX64/../../runtime/vm/service.cc:4892:34
[...]
    #31 0x5590af8a12ff in dart::ThreadPool::Worker::Main(unsigned long) ../../out/ReleaseASANX64/../../runtime/vm/thread_pool.cc:323:9

SUMMARY: AddressSanitizer: 256 byte(s) leaked in 1 allocation(s).

[mkustermann]

Several ASAN issues have surfaced after 519c222:

Don't think this commit can be related in any way to those asan failures.

This memory leak got introduced in b5c911e, which added a usage of Dart::FeaturesString() but didn't free the resulting string. I've made a fix in cl/216802