isFallbackDtsPath runs for every type lowering maps (isStdlibFile), and
since the fallback declarations moved into per-module files (#2256) it
asked declarationPath for the fallback module directory each time. Without
an installed declaration root, each answer resolved
@scriptc/compiler/scriptc-node-fallback.d.ts through Node's module
resolver. Lowering three.js's buffers-animation fixture went from 6.3 s to
43 s. declarationPath now keeps its answers by name until the declaration
root changes.
Chris Tate
f1298287d6