Fix @ember/string dependency #1867

Job	Run time
Lint files	3m 6s
Test addon (floating dependencies) (ember)	1m 14s
Test addon (locked dependencies) (ember)	58s
Test compatibility (ember-lts-4.4)	2m 17s
Test addon (floating dependencies) (node)	1m 18s
Test compatibility (ember-lts-4.8)	2m 17s
Test addon (locked dependencies) (node)	47s
Test addon (floating dependencies) (test-apps)	1m 10s
Test compatibility (ember-lts-4.12)	2m 21s
Test addon (locked dependencies) (test-apps)	45s
Test compatibility (ember-release)	2m 35s
Test compatibility (ember-beta)	2m 26s
Test compatibility (ember-canary)	2m 25s
Test compatibility (embroider-safe)	2m 40s
Deploy app	0s
	26m 19s

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