Memory leak (?)

Windows 7 x64, Node v9.8.0

https://github.com/thorn0/grunt-contrib-concat/tree/fast-source-maps
source-map is used in this file.

After updating to 0.7.3, I started to get this from time to time:

<--- Last few GCs --->

[8556:000000000046F060] 84622740 ms: Mark-sweep 1399.5 (1464.3) -> 1399.4 (1465.8) MB, 915.2 / 0.0 ms  allocation failure GC in old space requested
[8556:000000000046F060] 84623765 ms: Mark-sweep 1399.4 (1465.8) -> 1399.3 (1434.3) MB, 1024.9 / 0.0 ms  last resort GC in old space requested
[8556:000000000046F060] 84624527 ms: Mark-sweep 1399.3 (1434.3) -> 1399.3 (1434.3) MB, 761.8 / 0.1 ms  last resort GC in old space requested


<--- JS stacktrace --->

==== JS stack trace =========================================

Security context: 00000399B5425529 <JSObject>
    0: builtin exit frame: stringify(this=00000399B5408D59 <Object map = 0000034CDA482BA1>,000002A2EC202431 <String[0]: >,000002A2EC202201 <null>,0000024DA0883AC9 <Object
 map = 000001029AC8F251>)

    1: _write [d:\dev\srcui\grunt\concat\lib\sourcemap.js:215] [bytecode=000003D3792C0499 offset=174](this=0000031B39802289 <SourceMapConcatH
elper map = 000001A141A84E59>...

FATAL ERROR: CALL_AND_RETRY_LAST Allocation failed - JavaScript heap out of memory
 1: node::DecodeWrite
 2: node_module_register
 3: v8::internal::FatalProcessOutOfMemory
 4: v8::internal::FatalProcessOutOfMemory
 5: v8::internal::Factory::NewRawOneByteString
 6: v8::internal::Smi::SmiPrint
 7: v8::internal::StackGuard::HandleInterrupts
 8: v8::internal::wasm::LocalDeclEncoder::Size
 9: v8::internal::wasm::LocalDeclEncoder::Size
10: v8::internal::wasm::LocalDeclEncoder::Size
11: v8_inspector::protocol::Debugger::API::SearchMatch::fromJSONString
12: v8_inspector::protocol::Debugger::API::SearchMatch::fromJSONString
13: 000001E97AA86B21

Start at tasks/lib/sourcemap.js line 215 in the linked grunt-contrib-concat branch and inspect its source-map 0.7.3 usage. Reproduce the out-of-memory failure under Windows 7 x64 with Node v9.8.0, using the reported stack trace as the entry point. Done means identifying a reproducible regression and verifying its resolution.