When I woke up, I realized it was all a dream.

Literal

Eyes [obj-を] awakened-and-saw [discovery-と] everything [subj-が] dream [copula-である] [nom-こと] [subj-が] understood.

いっさい is the hiragana spelling of 一切. であること uses the formal copula in the nominalized clause. A slightly more literary rendering of the same 'it was all a dream' realization.